The present invention relates to a scanning probe microscope for measuring surface information on a sample with an atomic-order resolution for example.
Conventionally, a scanning tunneling microscope (STM) has been contrived as an example of a scanning probe microscope (SPM) by Binnig, Rohrer, et al. However, the STM can be used to observe electrically conductive samples only. Accordingly, there has been proposed an atomic force microscope (AFM, see Jpn. Pat. Appln. KOKAI Publication No. 62-130302) as an apparatus that utilizes the element techniques of the STM, including the servo technique, for the observation of insulating samples with an atomic-order resolution.
The AFM, which is regarded as an example of the SPM, comprises a cantilever 4, a scanner (e.g., a tube-type piezoelectric-scanner) 8, and a displacement sensor 10, as shown in FIG. 9, for example. The cantilever 4 has a pointed probe 2 on its free end. The scanner 8 supports the cantilever 4 and causes the probe 2 and a sample 6 to move relatively to each other. The displacement sensor 10 can, for example, optically detect deflection of the free end of the cantilever 4.
The cantilever 4, having its proximal end portion supported on a mount 12, is removably attached to a moving end of the scanner 8. The scanner 8 has its proximal end portion mounted on a specific base 14. The displacement sensor 10 contains therein an optical system, such as a laser oscillator or photosensor, and is attached to the moving end of the scanner 8 (first prior art).
When the probe 2 is brought close to the sample 6 placed on a stage 16, in this arrangement of the first prior art, the free end of the cantilever 4 is displaced by interactions (e.g., atomic force, contact force, viscosity, frictional force, magnetic force, etc.) between the tip of the probe 2 and the surface of the sample 6. Surface information (e.g., irregularity information) on the sample 6 or the like is measured three-dimensionally by relatively scanning the sample surface in the X- and Y-directions with the probe 2 while optically detecting the displacement of the free end (or feedback-controlling the scanner 8 in the Z-direction to keep the displacement of the free end constant).
Scanning probe microscopes having a scanner with improved scanning response are described in U.S. Pat. Nos. 5,463,897 and 5,560,244, for example.
Each of these scanning probe microscopes comprises a laser oscillator 18, a lens 20, and a photosensor (e.g., four-division photodiode) 22, as shown in FIG. 10A, for example. The laser oscillator 18 can deliver a specific laser beam into the scanner 8 through an aperture 14a that is formed in the base 14. The lens 20 can converge the incident laser beam, delivered from the oscillator 18 into the scanner 8, on the cantilever 4 (more specifically, on the back surface of the cantilever 4 opposite from that surface to which the probe 2 is attached). The photosensor 22 can receive reflected light from the back surface of the cantilever 4 when the laser beam is converged thereon.
The lens 20 is positioned and fixed to a mounting frame 24 in the scanner 8 so that the laser beam from the laser oscillator can be incident on the center of the lens 20. The photosensor 22 is supported on a support member 26 that is attached to the base 14. It can be moved parallel to its light receiving surface 22a (second prior art).
For other constructions, the second prior art resembles the first one. In the description to follow, therefore, like reference numerals are used to designate like portions, and a description of those portions is omitted.
In this arrangement of the second prior art, the laser beam emitted from the laser oscillator 18 is converged on the back surface of the cantilever 4 by the lens 20 after it is applied to the lens 20 through the aperture 14a in the base 14.
As this is done, the reflected light from the back surface of the cantilever 4 is applied to the photosensor (e.g., four-division photodiode) 22, and is converted into specific electrical signals (more specifically, electrical signals with intensities corresponding to the quantities of received light and/or the positions of light reception).
If the moving end of the scanner 8 is moved in the X- or Y-direction (e.g., in the X-direction) in this state, the cantilever 4 moves for substantially the same distance in the X-direction as the movement of the moving end. The attachment position of the lens 20 is adjusted in consideration of the focal distance so that the laser beam can be converged on the back surface of the cantilever 4 during X- and Y-direction scanning.
According to the first prior art, however, the cantilever 4, displacement sensor 10, and mount 12 are attached to the moving end of the scanner 8. Accordingly, the mass that acts on the moving end of the scanner 8 increases, so that the kinetic mass of the moving end increases. Thus, the resonance frequency of the scanner 8 is lowered, so that the scanning response for the X-, Y-, and Z-directions is lowered inevitably.
In measuring surface information on the sample 6 (e.g., semiconductor circuit pattern) that has a sharp stepped portion 6a, such as the one shown in FIG. 10B, for example, by means of the scanner 8 with low scanning response (or having a moving end with substantial kinetic mass), the displacement motion of the scanner 8 in the Z-direction cannot catch up with the scanning operation if the scanning speed for the X- and Y-directions is increased. Thus, it is difficult accurately to measure the surface information on the sample 6.
Further, the displacement sensor 10 is fitted with an adjusting knob for adjusting the relative positions of the aforesaid optical system and the cantilever 4, and the scanner 8 is subjected to a bending moment by means of an operating force that is applied to the displacement sensor 10 as the knob is manipulated. In general, the scanner 8 is formed of a thin ceramic material and is bonded to the base 14 with a specific adhesive agent. If the scanner 8 is subjected to the bending moment, therefore, it may be damaged or separated from the base 14, in some cases.
Although the second prior art is an improved technique that has been developed to solve the aforesaid problem of the first prior art, it is based on the sacrifice of some other technical effects of the first prior art.
Now let it be supposed that the moving end of the scanner 8 is displaced without any interaction between the sample 6 and the probe 2 (i.e., with the cantilever 4 kept distant enough from the sample 6).
According to the first prior art, the relative positions of the optical system of the displacement sensor 10 and the cantilever 4 are kept fixed without being influenced by the state of displacement of the moving end of the scanner 8, so that the position and angle of incidence of the laser beam on the cantilever 4 never change. In consequence, the position of application of the reflected light from the cantilever 4 to the photosensor 22 can be also kept constant. Thus, the electrical signals delivered from the displacement sensor 10 are always kept constant and never change.
According to the second prior art, on the other hand, if the moving end of the scanner 8 is displaced, the angle and/or position of incidence of the laser beam on the cantilever 4 changes depending on the state of displacement of the moving end.
In the case where the angle of incidence of the laser beam on the cantilever 4 changes, the position of application of the reflected light from the cantilever 4 to the photosensor 22 also changes. In consequence, the electrical signals delivered from the photosensor 22 have an output characteristic Z such that the free end of the cantilever 4 is displaced apparently by interactions between the irregular surface of the sample 6 and the probe 2, as compared with a displacement d of the moving end of the scanner 8, as shown in FIG. 10C.
In the case where the position of incidence of the laser beam on the cantilever 4 changes, on the other hand, the laser beam partially projects from the cantilever 4, so that the position of application of the reflected light to the photosensor 22 changes, and therefore, the light intensity distribution on the light receiving surface 22a changes. In consequence, the electrical signals delivered from the photosensor 22 have an output characteristic Z such that the free end of the cantilever 4 is displaced apparently by interactions between the irregular surface of the sample 6 and the probe 2, as compared with a displacement d of the moving end of the scanner 8, as shown in FIG. 10C.
If the surface information on the sample 6 is actually measured by the second prior art method, therefore, the electrical signals corresponding to the output characteristic Z are superposed as noise signals on the actual surface information (irregularity information) on the sample 6, so that it is difficult to measure the surface information accurately. The resulting surface information signal is not the simple sum of error signals that are produced when the free end of the cantilever 4 is distant enough from the sample 6. More specifically, the surface information signal is a function of the irregularity information on the sample 6 and the error signals.